Wilde Racing

iTrader: (26)

Well, I'm truly starting the process of my engine build, and though I want to stay close to my current set-up which seams to work well for me, I'm toying with a few ideas. One of which is a longer rod engine.
The engine will most likely be a 4.03 bore LQ4 block with the stock 3.662 crank. Static compression will be around 9.5:1 and peak boost will be 20 psi, unless I run out of turbo, then it will be supplemented with nitrous (no I don't have or ever used nitrous on this truck). Needless to say the pistons will be 2618. Possible coated...
I do not want to spin the engine up past 6,500 rpm, in fact I plain on shifting below 6000, but with the 4.10 gears, I will have to let it spin up in third to hit my goal trap speed. Plus on top of all that, I will be driving my truck almost daily. I hope to have a fuel-efficient daily driver by next spring, but I still would like to see 50,000 miles out of the engine. Also with the cylinder pressures that this engine will have, the top ring will be as far down as I can possibly get it. Which is the whole reason for this thread.

Is it really worth the trouble for all the benefits of a long rod at the possible cost of ring land material?

Blown06

iTrader: (32)

Here is the low down on rod length. Each application has its own school of thought, but for a street driven vehicle, I believe you will like as long of rod as possible up to a certain point. Most stock motors have a rod ratio of about 1.5 (give or take .1), therefore it is probably not cost effective to change the rod length in most situations. Remember you will have to go to a custom piston when changing rod length.

Longer Rod:
Advantages: Less side loading on the piston (less wear and oil consumption). More dwell time at BDC and TDC (better cylinder filling). Less frictional horsepower loss. More rotating mass (inertia).

Disadvantages: Custom (shorter pin height) piston must be used. If the piston is so short that the pin is "in the oil ring groove" you could have oil consumption issues. More rotating mass (slower acceleration). There is a point of diminishing returns. Rod ratio is calculated by dividing the stroke by the rod length. After about 1.7 the law of diminishing returns starts to take effect.

Short Rod: NHRA Pro stock motors have a rod ratio of about 1.2
Advantages: Less rotating mass (more acceleration),

Disadvantages: Custom (longer pin height) piston must be used, or excessive deck milling to get the compression height back up. Excessive side loading on the piston (more oil consumption, piston wear)

Rod ratio: 1.5-1.7 ideal and normal on most engines
< 1.5: Oil consumption, excessive side loading
>1.7: less frictional horsepower loss

just my .02

ZZebes

iTrader: (10)

I think that in my opinion i would go with the near stock rod length to facilitate the proper operation of the oil holes since that piston with 20psi and the gas is gonna be under ALOT of stresss

Wilde Racing

iTrader: (26)

You forgot a few "PROS"
Less rod angularity
Higher wrist pin location
Helps resist detonation
A lighter reciprocating assembly
Reduced piston rock
Better leverage on the crank for a longer time
Less ignition timing is required
Allow slightly more compression to be used
before detonation is a problem
Less average and peak piston velocity
Peak piston velocity is later in the down stroke

Also Durability goes up with a longer rod, Most endurance engines are using a
RL/S ratio of at least 1.9:1 and some as high as 2.2:1.
It "sounds" like a perfict match for a High boost engine to me, as long as there is enough meat on the piston for the rings...

TurboGibbs

iTrader: (6)

I wouldn't get a rod so long that it caused the wrist pin height to expose the lower oil ring. The increased use of oil from a high wrist pin isn't worth the advantages of a longer rod on a daily driver IMO. Not that the oil use would be excessive (maybe 1 qt every 1-2000 miles at most). Just a trade off. Increased piston costs, and a little more oil burn for the pros of the longer rod. It is arguable that the pros are worth it.

TurboGibbs

iTrader: (6)

just did some checking and the LS7 has a 1.52:1 rod ratio with a 6.067" rod and 4.00" stroke. Kinda wonder if GM shortened the rod from the std 6.098" to keep the wrist pin out of the lower oil ring?

zippy

Because you intend to boost it hard, I'd go with the stock length rod. The extra piston area over the long rod will be worth it since piston breakage is the most common failure in the short block when boost is applied.

Wilde Racing

iTrader: (26)

It sounds like most like the stock rod length. I was just wondering because the cost won't be that much diiferent. I'm going to be buying rods, and mostliklly custom pistons anyway.......

TouchOfEvil04

iTrader: (13)